1. Field of the Invention
The present invention relates to a method of treating a substance to be degraded and apparatus for its practice.
2. Description of the Related Art
In recent years, a large quantity of organic chlorinated compounds such as chlorinated ethylene and chlorinated methane have been used, as industrial technologies have been developed. Disposal of these compounds has become a serious problem, because their release into the environment as spent gases arises wide-spread contamination. To solve this problem, enormous efforts have been made.
One of the disposal methods involves, for example, adsorbing contaminants such as the organic chlorine compounds in an adsorbent such as activated carbon to remove the contaminants from the environment. The activated carbon into which the contaminants are adsorbed is regenerated by burning or heating with a steam, and is then recovered as a liquid containing the contaminants. Another known decomposition method involves degrading chlorinated ethylene using an oxidizer and a catalyst. Specifically, Japanese Patent Laid-Open No. 3-38297 describes a method of degrading the contaminants with ozone, and Japanese Patent Laid-Open No. 63-218293 describes a method of irradiating the contaminants with ultraviolet light under hydrogen peroxide. Also, it has been suggested to use sodium hypochlorite as the oxidizer. Japanese Patent Laid-Open No. 7-144137 describes a method of degrading the contaminants using light irradiation by suspending a photocatalyst comprising oxide semiconductor particles such as titanium oxide and liquid chlorinated ethylene under alkali conditions.
In addition, a photolysis method has been attempted in which ultraviolet light is radiated in a gas phase without using the oxidizer. Japanese Patent Laid-Open No. 62-191025 suggests a method of making an organic chlorine compound harmless by irradiating an exhaust gas containing this compound with ultraviolet light to provide an acidic decomposed gas and then washing it with an alkali solution. Japanese Patent Laid-Open No. 62-191095 suggests an apparatus for aerating a waste water containing an organic halogen compound, irradiating a discharged gas with ultraviolet light, and washing it with an alkali solution. Japanese Patent Laid-Open No. 8-257570 describes a degradation of chlorinated ethylene with iron powder. In this case, it may be a reduction degradation. As to the degradation of tetrachloroethylene (hereinafter referred to as “PCE”) using silicon particles, a reduction degradation of PCE has been reported.
It is known that chlorinated aliphatic hydrocarbons such as trichloroethylene (hereinafter referred to as “TCE”) and PCE can be degraded aerobically or anaerobically with microorganisms. It has been attempted to degrade or purify these compounds utilizing such a technique.
If the soil is contaminated with these various organic chlorine compounds, the environment is affected for a long period of time. There is therefore a great need to remove the contaminants and recover the soil.
Various decontamination attempts have been made. For example, the contaminants in the soil were suctioned using a vacuum pump and treated with activated carbon. Also, a bioremediation utilizing microorganisms and a degradation method employing ultraviolet light has been proposed.
In the vacuum suction method, the contaminants are removed through adsorption to the activated carbon. However, an adsorption efficiency of the activated carbon is low and it is difficult to treat substances with a high water content. In addition, the spent activated carbon into which the contaminants are adsorbed must be reprocessed.
In the bioremediation, degradation activity of the microorganisms is not necessarily stable, and controlling the degradation is sometimes relatively difficult. The problem of soil contamination by organic chlorine compounds is yet to be solved in practice.
To solve the above-mentioned problems, there is proposed an apparatus for degrading and purifying gaseous organic chlorine compounds by combining a chlorine-containing gas with the gaseous organic chlorine compound and irradiating the combined gases with light.
In such an apparatus, a chlorine gas produced from a chlorine-containing solution is used as a convenient and safe source for obtaining the chlorine-containing gas.
FIG. 1 is a schematic view of one embodiment. A means for producing a chlorine-containing air 11 comprises a water tank for storing a chlorine-containing solution 12, a pipe 13 for blowing an air to the solution, and a valve 14 for adjusting the amount of the air. The air passed through the chlorine-containing liquid contains the chlorine gas and is sent to a reaction vessel 5. A device for providing a substance to be degraded 1 introduces the substance into the reaction vessel 5. In the reaction vessel 5, the substance to be degraded is mixed with the chlorine gas-containing air. The mixed gases are irradiated with light for a predetermined time by a light irradiating means 4, whereby the substance is degraded. After the degradation, the gases are discharged via an exhaust pipe 6. As the chlorine solution in the water tank 12, there is used a solution having a hydrogen ion concentration (pH value) of 1 to 4 and a residual chlorine concentration of 5 to 150 mg/L. Such a solution can be obtained, for example, by dissolving a hypochlorite such as sodium hypochlorite and potassium hypochlorite in water. If the solution contains an inorganic acid and the like, the chlorine gas can be effectively produced.
Japanese Patent Laid-Open Nos. 9-299753 and 10-180040 disclose a light degradation apparatus utilizing a phenomenon that ultraviolet rays UV-B and C degrade a part of the contaminant.
The aforementioned apparatus for degrading and purifying the gaseous organic chlorine compounds degrades them by mixing with the chlorine gas provided from the chlorine-containing water. However, the contaminants are not rendered fully harmless. Rather, most of them are discharged as toxic substances. It is therefore required to further degrade and render harmless the degraded products after light degradation.
Environmental considerations are especially important in a recovery site of the contaminated soil. It is essential to prevent a discharge of the chlorine gas into the environment and purify the waste water.
As described above, various methods of degrading the organic chlorine compounds have been proposed. However, they may require complex devices for degradation and further processing to render the degraded products harmless. Accordingly, this leads to a conclusion that an art of degrading the organic chlorine compounds using an environmentally benign and practical apparatus without requiring special equipment is required. In other words, there is a need to provide a more convenient and efficient degradation method, and a degradation apparatus. Also, there is a need to provide an efficient degradation method without a treatment with activated carbon or microorganisms that does not result in a secondary contamination and produces less waste water, and an efficient degradation apparatus for using the degradation method.